In integrated circuit manufacturing, control of the various processes is key to achieving high yields, low scrap rates and low defect levels. For a process to function well in manufacturing, statistical process control monitoring methods are typically integrated into the process. Thus, high quality process designs are generally dependent upon the metrology methods used to statistically monitor and control the process.
Shallow trench isolation structures are generally formed on a semiconducting substrate, such as a silicon wafer. The upper surfaces of the substrate are typically coated with a fairly thin layer of a relatively hard electrically insulating material, which functions as a polish stop at a later point in the processing. Typically, silicon nitride is the material selected as the polish stop layer. The isolation trenches are then etched through the polish stop layer into the substrate. The trench is filled with an electrically insulating material, such as silicon dioxide. Sometimes the surfaces of the trench are then also coated with a polish stop layer.
As the insulating material is deposited in the trench, it is also typically deposited on the other upper surfaces of the substrate. Preferably, both the insulating material and the polish stop material are removed from all of the upper surfaces of the substrate, except from inside of the trenches. Most preferably, after the insulating material and the polish stop material are removed from these other upper surfaces, the top surface of the shallow trench isolation structure is planar with the top surface of the substrate surrounding the shallow trench isolation structure.
Direct chemical mechanical polishing is a typical process used to remove the insulating material down to the level of the polish stop layer. After the direct chemical mechanical polishing step, a wet etch process may be used to strip away the undesired polish stop layer. However, if the direct chemical mechanical polishing process does not remove all of the insulating layer from the top surface of the polish stop layer, and some of the insulating layer remains on top of the polish stop layer, then the stripping chemicals for the polish stop layer cannot reach those portions of the polish stop layer that are below the remaining insulating layers. This tends to result in residual polish stop material that is left behind on the substrate, when it should have all been removed. This residual material inhibits the subsequent formation of desired structures and decreases the manufacturing yield of the process. Thus, it is generally important that the shallow trench isolation structure be properly polished.
Conversely, when the isolation structures are over polished, the polishing process thins or “dishes” the insulating material in the trench. Furthermore, the polish stop layer may be thinned to the point that the shallow trench isolation structures are damaged. This damage may be caused directly by the polishing process, or indirectly by subsequent processing. If over polishing is severe, then portions of the polish stop layer may be completely eroded during the polishing, allowing the substrate below the polish stop layer to be damaged. Such damage decreases the yield of the manufacturing process, and thus increases the cost of producing the integrated circuits.
Different integrated circuit designs have different shallow trench isolation circuit patterns, which respond differently to a given amount of polishing. Thus, it is difficult to design a shallow trench isolation polishing process that works well for all designs. One factor that tends to affect the polishing rate of the isolation structures is the density of the structures, or in other words, how close the structures are to one another. Therefore, since the key to shallow trench isolation processing is the removal of all of the insulating material above the polish stop layer, but limited or no removal of the material within the trench, the polishing process must either be capable to correctly process all of the different circuit patterns, or adjusted on a pattern by pattern basis.
What is needed, therefore, is a system for readily determining how a given direct chemical mechanical polishing process affects the thickness of the insulating material layer on both high density shallow trench isolation structures and low density shallow trench isolation structures.